This application relates to optical fiber devices, and more specifically, to devices and sensing techniques using optical fibers to couple radiation energy.
Optical fibers can be used to guide and transmit optical signals. An optical fiber may include a high-index fiber core surrounded by a lower-index fiber cladding. This structure can confine the majority of energy of a guided optical mode within the fiber core and guide the optical energy to propagate along the fiber core through total internal reflections at the boundary of the fiber core and the fiber cladding. One method of coupling an optical signal into and out of a fiber is coupling along the fiber core axis. One distal end of the fiber may have a polished facet normal to the fiber axis to allow transmission of light out of the fiber along the longitudinal direction of the fiber core.
Since a fiber is compact in size and is mechanically flexible, it can be deployed in various environments to deliver and receive light where traditional light-guiding optical elements may be inconvenient or even infeasible. For example, a fiber may be directly coupled to a semiconductor diode or a semiconductor substrate containing optoelectronic circuits to receive light and deliver the received light to a desired destination. A fiber with an oblique-angle-polished end facet has been used to couple optical energy via evanescent fields through the angle-polished facet. See Ilchenko et. al., Optics Letters, vol. 24(11), pp. 723, 1999.
Optical fibers can also function as probes to collect optical signals for various sensing purposes. For example, a fiber tip may form a probe for a sensor to measure changes in a reflective surface's position. Some fiber-optical interferometers for displacement measurements use a fiber probe with a sensing facet that is perpendicular to the fiber core. A reflective surface whose displacement is under measurement (“measurand surface”) is generally oriented perpendicular to the fiber core. The fiber end facet and the reflective measurand surface form a Fabry-Perot cavity with the cavity optical axis along the fiber core. The output of this cavity is coupled back to the fiber and is sent to a photodetector for measurements. See U.S. Pat. No. 5,017,010 to Rugar et. al.